The present exemplary embodiments relate to media (e.g., document or paper) handling systems and systems for printing thereon and is especially applicable for a printing system comprising a plurality of associated xerographic devices or marking engines.
Printing systems including a plurality of marking engines are known and have been generally referred to as tandem engine printers or cluster printing systems. See U.S. Pat. No. 5,568,246. Such systems especially facilitate expeditious duplex printing (both sides of a document are printed) with the first side of a document being printed by one of the marking engines and the other side of the document being printed by another so that parallel printing of sequential documents can occur. The process path for the document usually requires an inversion of the document (the leading edge is reversed to become the trailing edge) to facilitate printing on the back side of the document. Inverter systems are well known and essentially comprise an arrangement of nip wheels or rollers which receive the document by extracting it from a main process path, then direct it back on to the process path after a 180° flip so that what had been the trailing edge of the document now leaves the inverter as the leading edge along the main process path. Inverters are thus fairly simple in their functional result; however, complexities occur as the printing system is required to handle different sizes and types of documents and where the marking engines themselves are arranged in a parallel printing system to effect different types of printing, e.g., black only printing versus color or custom color printing.
As a document is transported along its process path through the system, the document's precise position must be known and controlled. The adjustment of the documents to desired positions for accurate printing is generally referred to as a registering process and the apparatus used to achieve the process are known as registration systems. See U.S. Pat. No. 4,971,304, which is incorporated herein by reference. Precision registration systems generally comprise nip wheels in combination with document position sensors whereby the position information is used for feedback control of the nip wheels to adjust the document to the desired position. It can be appreciated that many registration systems require some release mechanism from the media handling path upstream of the nip registration wheels so that the wheels can freely effect whatever adjustment is desired. This requires a relatively long and expensive upstream paper handling path. In parallel printing systems using multiple marking engines, the required registration systems also adds to the overall media path length. As the number of marking engines increases, there is a corresponding increase in the associated inverting and registering systems. As these systems may be disposed along the main process path, the machine size and paper path reliability are inversely affected by the increased length of the paper path required to effectively release the documents for registration. Lateral paper registration requirements for containerized marking engines are challenging due to the need to accommodate both edge-registered and center-registered marking engines.
Another disadvantageous complexity especially occurring in parallel printing systems is the required change in the velocity of the media/document and/or desired sequencing, as it is transported through the printing system. As the document is transported through feeding, marking, and finishing components of a parallel printing system, the process speed along the media path can vary to a relatively high speed for transport along a highway path, but must necessarily be slowed for some operations, such as entering the transfer/marking system apparatus. Effective apparatus for buffering such required velocity changes and/or re-sequencing of the media also requires an increase in the main process path to accommodate document acceleration, deceleration, and sequencing between the different sections of the process path.
Especially for parallel printing systems, architectural innovations which effectively shorten the media process path, enhance the process path reliability and reduce overall machine size are highly desired. Additionally, it is desirable to have inverters that can do more than simply invert paper, for example, translate, deskew, buffer, re-sequence, and/or return media to a process path (inverted or non-inverted).
In normal operation, sheets will be fed into the high speed highway and taken off to either be printed or to be sent to a finishing device. Depending upon the arrangement of marking engines used, a sheet could travel a significant distance before it is diverted off the highway. Given the fact that sheet registration degradation is likely proportional to length of paper path traveled, it is believed that the sheet may have a significant amount of mis-registration by the time it exits the highway. At this point, the only registration devices are those currently designed into the input inverters.